(1) Field of the Invention
The present invention relates to a process for production of polycarbodiimide resin powder. More particularly, the present invention relates to a process for producing a polycarbodiimide resin powder simply in industry by cooling the reaction system when the system has reached a certain polymerization stage, to make the system a slurry.
(2) Description of the Prior Art
Polycarbodiimide resins are polymer compounds each having recurring units represented by --N.dbd.C.dbd.N--R--. Being superior in heat resistance, etc., they have been used in various applications such as molding material, resin modifier, adhesive and the like.
In general, polycarbodiimide resins, particularly aromatic polycarbodiimide resins which are excellent in heat resistance and chemical resistance, however, are sparingly soluble in various solvents and, when it is intended to produce them so as to have a high molecular weight, the reaction system becomes a gel, making it difficult to produce them in a powder form. The conventional techniques for producing a polycarbodiimide resin in a powder form include, for example, the followings.
T. W. Campbell et al., J. Org. Chem., 28, 2069 (1963)
C. S. Fold et al., Macromol. Syn., 1, 74 (1963)
T. W. Campbell et al., Macromol. Syn., 3, 109 (1969)
In these techniques, a polycarbodiimide resin powder is produced by using a single or mixed solvent capable of dissolving a raw material diisocyanate but incapable of dissolving a polycarbodiimide produced.
The polycarbodiimide resin powder obtained as a precipitate in the above technique, however, has a relatively low molecular weight and contains a large number of residual isocyanate groups. Therefore, when the resin powder is heated at temperatures of 180.degree. C. or higher, it melts and causes recarbodiimidization (which is decarboxylation), giving rise to foaming and swelling. This becomes a problem when the resin powder is used, for example, as a molding material or a modifier.
There was also mentioned about production of a polycarbodiimide resin powder of relatively high molecular weight in the above literature. In this study, gelation takes place in the course of the reaction, and the gel is ground in a poor solvent and subjected to solvent removal to produce a powder. This process is usable in a laboratory but is not practical because the steps for gel grinding and solvent removal must be employed and the poor solvent must be used in an excess amount relative to the gel, making the process inefficient.
In order to improve these problems, J. Appl. Polym. Sci., 21, 1999 (1977), Japanese Patent Publication No. 16759/1977, Japanese Patent Application Kokai (Laid-Open) No. 39223/1993, etc. disclose processes for reacting 4,4'-diphenylmethane diisocyanate and an organic monoisocyanate as a molecular weight-controlling agent in an inert organic solvent in the presence of a carbodiimidization catalyst to isolate a polycarbodiimide in a powder form.
In the process of J. Appl. Polym. Sci., 21, 1999 (1977), the above reaction is conducted in a xylene solvent; after the completion of the reaction, the reaction system is cooled to room temperature and the resulting solid precipitate is isolated by filtration; the filtrate is poured into an excess amount of hexane and the resulting solid precipitate is isolated by filtration; and the two precipitates are combined and dried to obtain a powder. In the literature, the powder is called a terminal-blocked polycarbodiimide and the properties are described.
The process of Japanese Patent Publication No. 16759/1977 is the same as that of the above literature except that a mixed solvent of benzene and hexane is used as a polymerization solvent as seen in Example 1.
In the process of Japanese Patent Application Kokai (Laid-Open) No. 239223/1993, the above-mentioned carbodiimidization is conducted using a halogen type solvent as a polymerization solvent to obtain a uniform polycarbodiimide resin solution; the solution is cooled and made into a slurry; the slurry is spray-dried to obtain a powder of narrow particle size distribution.
The features of these techniques are to (1) prevent gelation by the use of a terminal-blocking agent and resultant molecular weight control and (2) appropriately control the solubility of polycarbodiimide resin in solvent to precipitate a solid and powderize it.
In the thus-produced terminal-blocked polycarbodiimide resin powders, however, the terminal-blocking agents used react with each other because they are organic monoisocyanates, and produce monocarbodiimides and low-molecular weight polycarbodiimides. These products cause decomposition and gasification depending upon the application of the polycarbodiimide resin powder, inviting foaming, swelling, offensive odor, etc.
When the above polycarbodiimide powders are used in a medium such as oil, the low-molecular weight polymer portion of the powders may dissolve into the medium and may denaturate the medium. Moreover, organic monoisocyanates which are used ordinary are expensive and their use incurs an increased cost.